The Rhyacian-Orosirian basement at the northeastern region of the Araçuaí orogen, Bahia, Brazil: Further evidence of an island-arc accretion followed by syn- and post collisional plutons during the São Francisco-Congo paleocontinent assembly

Abstract

Granodioritic-tonalitic orthogneisses, together with granodioritic and high-K intrusions compound the Rhyacian-Orosirian basement units in the northeastern region of the Araçuaí belt, northeastern Brazil. These rocks were deformed and metamorphosed during the Neoproterozoic compressive event, but their genesis goes back to the Rhyacian-Orosirian times, when the São Francisco-Congo paleocontinent was amalgamated at the end of the Minas-Bahia orogeny (MBO). Field and petrographic data, twenty-two lithochemical analyses, three U-Pb (LA-ICPMS) zircon ages, zircon-in Lu-Hf and thirteen Sm-Nd analyses were conducted. Additionally, four previous U-Pb (SHRIMP) zircon ages were revised. The crystallization age of 2099 ± 14 Ma obtained for the granodioritic orthogneiss is close to the tonalitic-trondjhemitic granulites of the Buerarema complex in the cratonic interior (ca. 2.19 to 2.10 Ga). This fact, together with the similar lithochemical signatures, compatible with magnesian, calc-alkaline and sodic TTGs magmas, deficient in Nb, Ta, P, Ti, Y, Yb_N and enriched in La_N/Yb_N, suggest the extension of the Buerarema complex towards the interior of the Araçuaí belt, where it was reworked by the Neoproterozoic orogeny. The lithochemical signature observed for these orthogneisses is compatible with subduction-related magmas and the moderately juvenile signatures (ɛHf(t): −0.5 to −4.6; ɛNd(t): −1.6 to −4.6) suggest derivation from intraoceanic mature arcs. The presence of others ca. 2.1 Ga coeval arcs related to the Bahia-Minas orogeny (i.e.: the Rio Itapicuru, Costeiro, Rio Campim, Mineiro, Mantiqueira, Juiz de Fora and other arcs) not only corroborates with the interpretation of the pre-collisional Buerarema arc, but also suggest the presence of an expressive oceanic crust between the Archean blocks at ca. 2.1 Ga. One calc-alkaline and metaluminous granodioritic stock (2067 ± 5 Ma) and new mapped high-K shoshonitic granitoids (2025 ± 7 Ma) are intrusive in the Buerarema complex. Their isotopic record are indicative of crustal evolved rocks (ɛHf(t): −4 to −7; ɛNd(t): −2.8 to −3.0; and ɛHf(t): −7 to −14; ɛNd(t): −2.4 to −2.6, respectively) compatible with syn and post-collisional magmas of the MBO. Although the Paleoproterozoic age, the high content of Ba-Sr in concordance with the elevate ferromagnesian minerals and incompatible trace and REE elements allow the discussion if these intrusions are sanukitoid-like rocks.